html/template: make FuncMap a type alias of text/template.FuncMap

html/template: make FuncMap a type alias of text/template.FuncMap

In preparation to make html/template and text/template use a
single generics package, this change make FuncMap a type alias
of text/template.FuncMap.

Fixes #46121

Change-Id: I5e94cc8fdf4fe029ff223992b5cdfe79f9c098ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/389156
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>